In new mobile-phone communication standards such as LTE (Long Term Evolution) for which the standardization work has been carried out by the standardization organization 3GPP (3rd Generation Partnership Project), data communication technologies have been developed for enabling high-speed communication at the mobile terminal.
As a retransmitting technique adopted in LTE and the like, a system called Hybrid Automatic Reseat reQuest (HARQ) is known. This system, which has been applied to HSDPA (High Speed Downlink PACKet Access), performs error detection for a communication packet received by the receiver based on an error correcting code attached to the communication packet at the transmitter. Then, the receiver notifies of the transmitter the success/failure of the reception of the communication packet by returning ACK (ACKnowledgement) or Nack (Negative ACKnowledgement) to the transmitter. Therefore, the transmitter can perform retransmission of data by detecting that the data transmission was not performed normally, by reception of Nack.
Specifically, the transmitter transmits data (code block) in which the information bit is encoded, as new transmission data to the receiver. Meanwhile, data to be transmitted is stored at a buffer (HARQ buffer) at the time of new transmission. The transmitter retransmits the transmission data when the receiver returns Nack or when no acknowledgement is received by a reasonable time passes since the packet was transmitted. In this case, the transmitter can also select and transmit a transmission bit from the HARQ buffer according to a fixed rule, upon consideration that the data for which decoding was unsuccessful at the receiver is not discarded and is decoded in combination with the retransmission data. At this time, the same bit as at the time when the new data was transmitted does not need to be transmitted. Meanwhile, the method in which the exactly same bit as at the time of new transmission is transmitted with the retransmission is called Chase combining, and the method in which a different bit is transmitted is called Incremental redundancy.
The receiver side can, even if reception was unsuccessful, perform decoding without discarding the reception data and combining (compounding) it with retransmission data.
There are prior-art documents as stated below as related conventional arts.
[Patent document 1] Japanese Laid-open Patent Publication No. H5-160817
[Patent document 2] Japanese Laid-open Patent Publication No. 2005-109909
[Patent document 3] Japanese Laid-open Patent Publication No. 2006-203355